An often used procedure for the chemical or physical alteration of material involves the passage of such material through a bed of adsorbent, catalyst, etc. depending on the intended result. When a continuous process is employed, a packed bed presents certain problems. While it is desired to have short cycle times in order to minimize bed inventory and equipment size, longer cycle times are generally more energy efficient. Furthermore, there is a need for an elaborate and expensive valve assembly for cycling various streams among various beds, which can lead to increased maintenance costs. These operating problems become more acute as one tries to minimize capital expense by reducing cycle times. Those skilled in the art have long recognized the advantages of moving and fluidized beds over packed beds. Such advantages include a greater energy efficiency because of the reduction of cyclic and transient energy losses.
A problem with the use of certain particles in moving beds is attrition of the particle by abrasion of its surface. In fixed beds, fluid flow may cause adjacent particles to contact and abrade each other, especially when a localized portion or the entire bed is accidentally fluidized. Movement may also be caused by external forces such as vibrations due to a nearby compressor or location of the bed on a moving vehicle. However, in moving and fluidized beds the problem of attrition is greatly magnified. Excessive particle attrition is caused, for example, by abrasion among bed particles, abrasion with bed walls and bed internals and distributor jet impingement, and abrasion in circulation conduits to and from the bed. High particle attrition contributes to product contamination, particle loss, plugging of downstream equipment, high filtration costs, and unstable fluidization behavior such as channeling, slugging or increased entrainment.
The problem of particle attrition is especially severe with high porosity bed particles such as molecular sieves. Molecular sieve beads or pellets consist essentially of zeolite crystals and a clay binder material. Due to the ceramic nature of both these materials, the surface is highly abrasive and subject to attrition. The amount of surface attrition caused by an impact on the particle depends on the particle's momentum, which is the product of its mass and its velocity. Therefore, smaller particles traveling at low speeds, i.e. in a bed having low fluidization velocity, do not suffer as much attrition as large, highly fluidized particles. The total amount of attrition, as measured by the amount of dust generated in the moving bed, includes surface attrition and attrition due to the breaking up of the entire particle.
It has been possible to employ molecular sieves in fixed beds without excessive attrition. However, it has not generally been economical to use molecular sieves for moving beds, except in very specialized applications such as cracking petroleum fractions for gasoline. In this approach, very small molecular sieve particles made up of about 80% clay binder are formed. Molecular sieves employed in packed beds generally have only about 20% clay. The particles containing mostly clay have a higher crush strength, but a lower mass transfer efficiency, than the conventional friable molecular sieve particles. In catalytic cracking, the granular type particles are swept along by a carrier gas with which they react. The attrition experienced is somewhat less than that of particles containing 20% clay, and is nearly equal to the amount of sieve that must be replaced anyway due to loss of reactivity, so the attrition does not prevent the use of these particles in a moving bed. The slight reduction in total attrition is belived to be largely attributable to fewer particles becoming pulverized, as opposed to any significant difference in surface attrition of the "harder" particles.
Thus, the capability to employ friable molecular sieves in moving and fluidized beds while keeping attrition low without a significant reduction in efficiency would be highly desirable.
Another problem in the operation of moving and fluidized beds is the need for efficient movement of the bed particles relative to one another and also relative to the stationary bed apparatus. As a general rule, the greater the flowability of the bed particles, i.e. their ease of motion through the bed system, the greater is the efficiency of the bed process, be it a chemical reaction, a physical change such as adsorption, or merely a transport of mass.
A recent significant breakthrough in this field has been the development of an attrition resistant molecular sieve particle which exhibits increased attrition resistance when employed in a moving bed without a significant decrease in activity. This attrition resistant molecular sieve particle also enables the moving bed to be operated with improved efficiency due to greater bed particle flowability. This attrition resistant molecular sieve particle and the method of making it are disclosed and claimed in U.S. Pat. No. 4,526,877--Acharya et al.
U.S. Pat. No. 4,526,877 discloses three methods of making the attrition resistant molecular sieve particles. One method involves rolling sieve particles in a drum or other roll mill means containing solid lubricant, preferably in powder form. A second method involves mixing molecular sieve particles with solid lubricant-supplying pellets in a moving bed and operating the bed. A third method involves spraying molecular sieve particles with a liquid suspension of solid lubricant and subsequently drying the particles.
While all of these methods are effective, each method has disadvantages. The rolling mill method has the major disadvantage of either requiring a temporary halt to the moving bed process while the bed particles pass through the rolling mill operation, requiring an excessively large particle inventory in order to enable continuous operation of the moving bed or requiring the coating of the particles with more than the minimum coating so that the particles need not be as frequently removed from the bed for recoating. The solid lubricant-supplying pellet method has several disadvantages. It is generally very difficult to match the physical characteristics, such as size, density, shape, etc., of the pellet to the moving bed particle. The presence of solid lubricant-supplying pellets and moving bed particles having dissimilar physical characteristics in the same moving bed system could result in unstable and/or unpredictable moving bed behavior. Furthermore, the solid lubricant-supplying pellet may not be compatible under moving bed particle regeneration conditions such as high temperature. The liquid suspension method has the disadvantages of requiring a relatively long time and complicated procedures to be effective, and may not be useable where the liquid impairs the structural or mass transfer efficiency of the moving bed particles.
It is therefore an object of this invention to provide an improved moving bed apparatus and process.
It is another object of this invention to provide an improved moving bed apparatus and process wherein the moving bed particles are rendered more resistant to attrition without the need for applying solid lubricant to the particles in a separate rolling mill.
It is a further object of this invention to provide an improved moving bed apparatus and process wherein the moving bed particles are rendered more resistant to attrition without the need for employing solid lubricant-supplying pellets in the moving bed system.
It is yet another object of this invention to provide an improved moving bed apparatus and process wherein the moving bed particles are rendered more resistant to attrition without the need of applying a liquid suspension of solid lubricant to the particles.
It is a still further object of this invention to provide an improved moving bed apparatus and process wherein the moving bed particles move within and throughout the moving bed system with improved flowability.